The present invention pertains to gas turbine rotor apparatus and, more particularly, to blade flow path defining platforms for use therein.
Many rotor blades of dynamic machines such as axial flow gas turbine engine compressors and fans employ platforms extending generally laterally of the blades to partially define the aerodynamic flow path between adjacent blades. In the case of metallic blades which are retained for rotation upon the periphery of a rotatable disc, it is common to find the blade platforms cast or forged integral with the blades, laterally adjacent platforms abutting to define the flow path. Alternatively, platforms may be formed integral with the disc. In other embodiments, platforms do not entirely span the gap between adjacent blades, and flow path defining spacers are provided to fill the voids.
The trend toward incorporating composite blades into gas turbine engines has produced unique problems not heretofore experienced. By the term "composite blades" it is meant those blades formed by laminating multiple plies of elongated, small diameter filaments of high strength (high modulus of elasticity) embedded in a lightweight matrix. Typical examples are the nonmetallic composites such as graphite filaments in an epoxy resin, and the metallic composites represented by boron filaments embedded in an aluminum matrix.
One significant problem associated with composite blades is their relatively low tolerance to foreign object impact. For example, objects such as stones, ice or birds may be entrained in the airstream entering the engine inlet and impacted by the rotating blades. While damage by smaller objects may merely result in blade erosion, impact by larger objects may rupture or pierce the blade, causing failure of the blade and possible secondary damage to downstream components. This vulnerability of composite blades to foreign object damage is due to two factors. First, the lightweight matrix materials employed are relatively soft. Second, the high strength filaments are relatively hard and brittle such that the blade is incapable of significant bending without filament fracture, delamination of adjacent plies, or pullout of the filaments from the matrix.
A partial solution to the problem is to provide the composite blade with what has become known in the art as a "swing root." In such a blade, the root portion is substantially cylindrical and is received within a similarly contoured, cooperating groove in the disc such that the blade is free to swing or rotate laterally within the groove if impacted with a lateral force. Centrifugal force keeps the blade in an upright or radial position under normal operation. Since the blade is free to swing, bending stresses are eliminated and the possibility of failure reduced.
However, it becomes apparent that the typical approaches to blade platforms are no longer appropriate for swing root blades due to the relative motion and possible interference between the blade and adjacent flow path structure. The problem is further compounded if the blades are of the variable pitch variety (i.e., rotatable about the blade longitudinal axis) since an additional degree of movement is added to the system. Further, during variable pitch operation the platform may become misaligned with the adjacent flow path, providing large cavities and areas for flow disturbance. Thus, it becomes necessary to provide a unique blade platform for blades of the swing root variety, adaptable also to swing root, variable pitch blades. Preferably, the structure should minimize the potential flow leakage between relatively movable components since such leakage is directly related to rotor inefficiency.